This invention relates to electric motors of the type having a fixed part, a part rotatable about an axis, a winding incorporated in one of these parts and including an assembly of active conductor portions distributed about the axis at a given pitch, and a feed device which is normally connected to a constant-voltage current source and controls, at a frequency corresponding to the pitch of the conductor portions, commutations of the connections between these portions and the poles of the current source.
In motors of this type, the feed device may be either a commutator, if the winding is incorporated in the movable part, or an electronic device if, on the contrary, the winding is incorporated in the fixed part. In both cases, the design of the feed device and of the winding assembly, as well as of the magnetic flux or fluxes must take into account that the motor torque varies periodically, the difference between its maximum and minimum values being proportional to 1-cos (.pi./2n), 2n being the number of commutations per pair of poles. In order to reduce the value of this difference, n may be increased, but this creates structural complications.
Another solution to this problem has been described in an article by J. Lindner which appeared in the journal Feinwerktechnik und Messtechnik, No. 88 (1980), p.4. This solution consists in eliminating the side portions of the coils.
Published U.K. Patent Application No. 2,000,646A describes a DC motor design in which the rotor comprises permanent magnets and the stator comprises fixed coils distributed about the axis. This likewise shows that the variation of the motor torque can be reduced during rotation by providing a dipole arrangement on certain angular zones of the rotor and giving the coils a polygonal outline. In this prior art design, however, the desired result is achieved at the cost of an incomplete filling of the coils and, consequently, a reduction in the power supplied.
It is an object of this invention to provide an improved electric motor arranged to reduce considerably the undulation of the torque without increasing the number of commutator segments and without limiting the filling of the coils, based upon the fact that a curve which is sinusoidal or is formed of portions of sinusoids can be "flattened" by adding to the mathematical experssion of the curve terms representing undulations having a shorter period than that of the basic sine curve, such periods being odd-number sub-multiples of the basic period. In other words, the shorter period is a fraction of the basic period determined by dividing the basic period by the odd-number.
To this end, in the electric motor according to the present invention, of the type initially mentioned, the winding comprises, besides the active conductor portions, auxiliary portions formed of sections of conductors disposed in series with active conductors and having an orientation relative to an axis of rotation similar to that of the active conductors with which they are in series, the auxiliary portions being distributed at a pitch which is an odd-number sub-multiple of the pitch of the portions of active conductors. In other words, the angular pitch of the auxiliary portions is a fraction of the angular pitch of the active conductor portions determined by dividing the angular pitch of the active conductors by the odd-number.
The effect of this arrangement is that upon rotation of the motor, an induced voltage is produced in the auxiliary portions, the frequency of which is a multiple of the voltage induced in the normal part of the winding, this multiple being an odd number equal to the above-mentioned sub-multiple.
In one preferred embodiment, in which the winding is incorporated in the part of the motor which is rotatable and is formed of flat coils distributed in the shape of a disk, a portion of each coil situated toward the coil heads forms a fret or meander which, when there are three pairs of auxiliary portions, occupies a sector of 120 electrical degrees.